The present invention is directed to a method and apparatus for measuring steam quality of a hybrid stream having both vapor and liquid phases directly in the flow line.
One of the least expensive and potentially most cost effective methods of enhanced oil recovery (EOR) involves the injection of high pressure steam to heat the oil reducing its viscosity and to thereby displace the oil from the pore volume of the oil-bearing formation entrapping it. In order to properly estimate the amount of oil that can be recovered through EOR, one must know the volume and the quality of steam in order to calculate the quantity of heat input into the reservoir.
In the steam injection process, single path steam generators are typically used. Dry superheated steam (i.e., quality of 100%) is not used in this process because of the potentially harmful effects to the equipment. The impurities in the water will not remain in solution in dry steam and build up forming scale in the steam generator and associated feed lines. Scale buildup insulates the tubes of the steam generator preventing proper heat dissipation leading to overheating and possible rupture at high pressures. Steam having a quality of 80% (i.e., 80% by weight vapor, 20% liquid) has been shown to be optimum, protecting equipment from scale buildup while providing an acceptable rate of heat input. Without an adequate in-line means of determining steam quality, operators will typically run their systems at lower quality steam output to protect their steam generators. This results in lesser quantities of heat being input to the reservoir resulting in less oil being recovered than calculated and potentially jeopardizing the economics of the EOR process. Further, with a fluctuating steam quality, the operating efficiency of the steam generator remains an unknown, although the operating life will certainly be less than it could be were a steady state steam quality to be maintained.
One of the most significant problems associated with steam injection for EOR applications is the absence of any means to directly measure steam quality in the flow line. Currently, one of the most widely used processes involves titration of an extracted liquid sample to compare the percentage level of impurities in the input water used to formuate the steam. This process is time consuming and inhibits accurate steam quality control. Further, if a single steam generator is providing steam to a plurality of injection wells by means of stream splitting using conventional piping such as a pipe tee (a process for which it can be demonstrated that the qualities of the resultant streams are not uniform), the titration process cannot be used to determine steam quality.
Orifice meters can be used to estimate steam quality if flow rate is known or to estimate flow rate if quality is known. Steam flow rate is related to steam quality by the following formula: EQU M=a.multidot.b(f1)(f2).sqroot.pP
where:
M=mass flow rate of steam in pounds per hour
a=basic orifice factor, a constant for a particular pipe and orifice diameter
b=an expansion factor, also a constant for a particular system
f1=an empirical correction factor for steam quality
f2=correction factor to account for expansion through orifice plate
p=differential pressure across the orifice
P=absolute flow line pressure
The reason orifice meters produce estimates rather than accurate measurements is two fold. First, the presence of significant amounts of liquid (i.e., exceeding 5% by weight) in the flow stream make it impossible to obtain accurate readings of the differential pressure p. A best guess average is generally used. Second, the estimate is derived through an iterative process in which various of the factors are derived through interpolation and then adjustments are made to the results to achieve the desired mass balance on the two sides of the equation. Such an approximation is not entirely satisfactory.
The method and apparatus of the present invention enable a direct real-time measurement of steam quality to be made. The apparatus of the present invention utilizes an in-line, high-frequency (e.g., 40 MHz) capacitance cell. The capacitance of the cell is a function of the dielectric constant of the material within the cell which varies with the percentage of liquid water (i.e., with the steam quality). The change in capacitance of the cell alters the resonant (or resultant) frequency of the electrical circuit of which it is a part. It is preferred that the resultant frequency from the cell be mixed with a reference frequency to produce a beat frequency. This enables more accurate readngs to be made and less sophisticated equipment to be used to measure the capacitance changes resulting from the changes in steam quality.
Various other features, advantages and characteristics of the present invention will become apparent after a reading of the following specification.